The present invention relates to continuous wave ultrasound imaging. In particular, transmitters and channel count reduction are provided for steered continuous wave Doppler ultrasound imaging.
To reduce the channel count, signals from different elements are combined to form subarrays. The use of subarrays may minimize the number of receive beamformer channels used in an ultrasound imaging system or the number of cables to communicate the signals from the elements to the ultrasound imaging system. For example, a two-dimensional transducer array is divided into a number of pre-set subarrays. Signals from elements within a same subarray are combined together and transmitted through a cable to an ultrasound imaging system. The number of subarrays corresponds to the number of cables and receive beamformer channels of the imaging system. However, the subarrays may change with different steering.
U.S. application Ser. Nos. 10/741,827 and 10/741,538, the disclosures of which are incorporated herein by reference, disclose altering the size of subarrays as a programmable trade off between the number of transducer array elements and the number of receive beamformer channels. The size, shape or location of subarrays may be dynamically varied. Electronics are provided in the transducer assembly for implementing the subarray variation. The electronics provide time division multiplexing or partial beamforming for the subarrays. The electronics allow variation in subarray size for use with a same ultrasound imaging system.
Time division multiplexing or partial beamforming are implemented with clocked circuits or switching applied to receive signals. Other partial beamforming within a transducer probe includes charge coupled device delay lines or linear phased switched capacitors. Switching limits the dynamic range of the signal. For pulsed wave imaging, the dynamic range limitations may be acceptable. However, for continuous wave imaging, the limits on dynamic range from switching may be unacceptable.
Switching also dissipates power. Power consumption for switching may result in less power for preamplification. Power is also used to continuously generate waveforms. Less power for preamplification reduces the amount of available dynamic range.
Similarly, the transmitters used for pulsed wave imaging use high voltage sources with a low duty cycle. The transmitters are also used for continuous wave imaging by reducing the voltage, resulting in poor power dissipation. Using a large number of high voltage transmitters for continuous wave transmission with a multidimensional array exacerbates power dissipation problems.